clonal propagation of walnut rootstock genotypes for genetic ...

Bench grafting root cuttings: A small root-grafting experiment was performed to comparerooting of root pieces when the temperature was kept at 78ºF at either the graft union only or atboth the graft union and the base of the root piece. As shown in Table 5, rooting was as good orbetter for three of four clones tested when heat was provided at the graft union only. WithAZ025, a sparse rooting clone, providing heat to the base did improve rooting somewhat. It maybe that the heating cable at the graft union provides a temperature at the base, even thoughprobably lower than that at the graft union, high enough to promote rooting. This suggests thatthe optimum temperature for rooting may be lower than that for graft union healing (however, notemperature measurements have been made to confirm this). These results do indicate thatproviding heat to the base of root grafts may not be necessary for successful rooting of mostclones and may even be detrimental.Due to concern that the root pieces used to make root grafts have been in contact with field soiland that the wounding necessary to the process could increase the incidence of crown gall on theresulting nursery trees, we evaluated 129 root grafted trees after two years in the nursery row.Seven of the 129 trees had galls (5.4%) at the site of the graft union. This is a very highincidence of crown gall and may indicate that root grafting can exacerbate nursery problems withcrown gall. However, of the six rootstock clones used for the root grafts, WIP3 trees had five ofthe seven galls detected. The frequency of galls on WIP3 trees was 36%. The frequency on theother five clones was 1.8% which is still quite high. The high incidence for WIP3 trees is basedon a small sample but other data based on inoculation of WIP3 trees with a virulent strain of A.tumefaciens also indicate that it is quite susceptible to crown gall formation.Field Trials: Currently established clonal rootstock field trials are summarized in Table 6.Fully chilled liner plantlets (100 each) of Vlach, VX211, RX1 and Burbank were provided toSuchan Nursery for growing to sufficient size for planting as dormant bare-root trees in anorchard trial to be established at Cilker Orchards in Yolo County in 2009 and grafted withHoward scions.The largest orchard trial of clonal rootstocks to date was planted this year in San Joaquin Countyunder the direction of Joe Grant. Chandler-grafted nursery trees using five rootstock clones andseedling paradox were dug in January 2008 and planted along with own-rooted Chandlers.A one acre field plot at UC Davis was planted with transgenic, putatively crown gall resistantlines and controls for assessment of horticultural characteristics.Nursery Propagation and Commercialization: We are prepared to provide cultures ofmicroshoots to any laboratory or nursery that wants them for licensed production of plants. Wecan also provide microshoots of Vlach, a public domain clone, to any laboratory or nursery thatwants to produce it. The appendix includes a list of laboratories presently licensed for in vitroproduction of clonal rootstock selections and sale of clonal rootstock plantlets as liners fornursery or orchard planting.California Walnut Board 66 Walnut Research Reports 2008